Begin the process by selecting your subject for the book. Find a person who is interesting to you. Make sure the person has had an unusual background that will appeal to an audience. Check that there are enough experiences to cover the amount of space allotted. Your biography can be about a famous person or an ordinary person who has an extraordinary story to relate.
Gather your information. Look for newspaper accounts about the person in your biography. Check other biographies, reference books or histories that provide you with specific information about the person. Find out about his/her accomplishments. Try to arrange an interview with the person himself if he is still alive. Attempt to use other sources, if available. For example, if he's deceased but frequented a particular restaurant, go there if feasible and interview the owner. Contact an organization that he belonged to and ask for information. Try to find a surviving relative who might grant you a telephone interview.
Focus on analyzing and interpreting events in the particular person's life. Think about what you'd like to know about the person. Ask yourself, what was significant in her life? Think about the effect the person had on the world or other people. Try to select an aspect that hasn't been told. Look for items of historical significance related to the subject of the biography or incidents that might lend themselves for a pleasant read.
Decide what structure you want to use for the biography. Most authors will use chronological order, although others use professional development. Make an outline. Divide your work into chapters covering the material you have uncovered. Think of a thesis sentence for your major points. This will tell the reader what you are trying to express about the subject of your biography.
Using your outline and chapter divisions for your biography, write everything that comes to mind. Write more rather than less since you can delete as you go along. Use your thesis statement as a guide and develop your material about the specific person in your biography. Keep focused on the main idea and then fill in specific details.
Check that you have cut extraneous material from the biography. See that you have developed your thesis statement and told enough ideas about the person you are describing so that the reader will obtain a vivid portrayal of your subject. Make any necessary revisions. Check for spelling and grammatical errors. Consider sending your manuscript to a few publishers for consideration. Look for appropriate places on the Internet or in "Writer's Market." If you don't find a publisher, think about self-publishing, where you will pay the costs, but you will have a biography book to show for your efforts.